Instead of a regular pair of pliers, see if they will unscrew with a pair of vise grip pliers (see pic below). If not, they're rivets as mentioned.

It could always be dremmeled off. I would hate to do that because it's there for a reason and you may screw up the frame in another way.

Another idea; That security bracket looks like it's bent around the metal frame. Can't tell by the pic, is it possible to remove the metal frame where it's attached and see if it can be removed that way. Looks like a couple of regular Philips head screws.

If all attempts fail, see if you can just bend that piece back where it was so the window closes again and call it a day. Just a couple more ideas.